meteorites from Mars

More than 100 meteorites have been found on
Earth that have come from Mars, having been ejected
from that planet millions of year ago by asteroid collisions. All are igneous rocks that formed by crystallization of cooling magma in the martian crust. As such all are achondrites formed by differentiation in their parent body rather than primitive chondrites
formed by accretion of chondrules in the early solar nebula. Martian meteorites
have many similarities to HED achondrites and were grouped with HED meteorites until their young crystallization ages
and distinct oxygen isotope trend defined them as a distinct group known
as SNC meteorites. This group includes the shergottites, nakhlites, and chassignites.

The SNC group was named after the type samples of the first three rock types: basalt Shergotty, clinopyroxene cumulate Nakhla, and olivine cumulate Chassigny. As more samples of the group were identified, some of
them fitted well into the existing types; for example, Zagami and EETA79001
as shergottite basalts and Lafayette and Governador Valadares meteorites as nakhlite cumulates. ALHA77005, however,
was found to be related to shergottites, but be a cumulate rather than a
basalt. It was nonetheless classified as a shergottite. By 1985 the discovery
of martian atmosphere trapped inside EETA79001 had convinced many meteoriticists
that the SNC meteorites were from Mars.

Family groups

There are now five distinct igneous rock types in the martian meteorite
family. Bulk compositions are generally consistent with mineralogy. Shergottites
are divided into pyroxene-plagioclase basalts (S-B) and pyroxene-olivine
lherzolites (S-L). Nakhlites (N) are clinopyroxene cumulates. Chassigny
(C) is an olivine cumulate or dunite and ALH84001 is an orthopyroxene cumulate
(O). The first three types are populated by multiple samples, ALH84001 and
Chassigny are singular members of their classifications. Because Mars is
a complex planet one can expect that as more martian meteorites are identified,
new rock types will continue to be found.

The most famous
martian meteorite

ALH84001, which became the center of a
major controversy in 1996 when a group of NASA scientists claimed it contained fossils and other biological remains, was
initially misclassified as a diogenite (orthopyroxene cumulate) because it fitted well into that mineralogical
classification and because the SNC meteorites were presumed to be limited
in their lithologies. In 1993 it was found to contain oxidized iron in its
accessory oxide phases, in contrast with the reduced iron in the oxides
of HED meteorites. This implied that ALH84001 came from a volatile-rich
Earth-like planet rather than a volatile-poor asteroid. Oxygen isotope analyses
proved that it was a part of the SNC family even though it is not a member
of any of the three types.

More recently two other martian meteorites, Nakhla and Yamato 593, have
been found to contain what appear to be biomorphs (suspected fossil forms)
and other evidence for past life, including chains of magnetite crystals
and carbonates deposited from at moderate temperatures. The age spread of
ALH84001, Nakhla, and Yamato, from 3.6 billion to 1.4 billion years ago,
suggests that life was present on Mars throughout that period, and may still
exist there today. See martian fossils controversy and life on Mars.

From
Mars to Earth

In The War of the Worlds,
H. G. Wells imagines the Martians firing their invading ships Earthward
out of a great cannon (much like the Columbiad of Jules Verne's
lunar classic). But could a natural collision similarly have blasted pieces
of Mars into space? Photos of the martian surface taken by the Viking orbiters
revealed scores of craters that are obviously the result of oblique impacts.
One of these is a crater measuring 14 by 31 km, located at the southern
approaches to a volcano known as Ceraunius
Tholus, about 950 km west of the Viking 1 landing site. Conceivably, some
of the lava that flowed out of this volcano, 1.3 billion years ago, cooled
to form the basalt contained within EETA
79001, although a number of other craters have also been identified as possible
sources of the rock.

The mechanics of how SNC meteorites might have been ejected were first investigated
in detail in the early 1980s by Laurence Nyquist at the Johnson
Space Center in Houston. A large object striking Mars at 35,000 kilometers per
hour could, he showed, hurl material from the surface at a speed in excess
of the 18,000-kilometers per hour martian escape velocity.
In fact, his calculations revealed, the escape velocity would be easily
surpassed if heat from the impact vaporized permafrost below the martian surface and so gave rise to a massive gas explosion. These
conclusions were confirmed by the work of John O'Keefe and Thomas Ahrens
at the California Institute of Technology in 19861 (see ballistic
panspermia).